1. Field of the Invention
This invention relates to a method for optically assaying liquid samples. More particularly, it relates to a method for assaying extremely small amounts of samples for their optical properties with high precision.
2. Description of the Prior Art
It is known that when a reaction is conducted in a cell for use in optical measurement the change in an optical property such as optical density of the sample with the progress of the reaction can be followed by an optical means to learn the progress of the reaction. This technique has been applied to various assay systems, for example, assay of an antigen or antibody by its antigen-antibody reaction with a sensitized latex, which system has recently been in the limelight. In a typical antigen-antibody reaction, a sensitized latex comprising insoluble carrier particles having an average diameter of not greater than 1.6 microns on which the appropriate antigen or antibody has been supported is placed in a cell for optical measurement and an antibody- and/or antigen-containing sample is added to cause the desired antigen-antibody reaction with stirring. The cell is irradiated with light of wavelengths in the range of 0.6 to 2.4.mu. and greater than the average diameter of the latex particles by a factor of at least 1.1 and preferably at least 1.5, and the change in an optical property of the latex, usually amount of transmitted light, is measured. In this way, the sample can be assayed usually by determining the velocity of change in absorbance or transmittance, the absorbance after a given period, or the time required to reach a given absorbance (see U.S. Pat. No. 4,118,192). In such assay of antigen-antibody reactions, it is necessary to agitate the liquid in the cell with adequate vigor in order to keep the content of the cell uniform and accelerate the reaction. A conventional stirrer occupies only a small part of the cell volume and is positioned out of the path of irradiated light. If such stirrer is positioned within the light path, it is usually removed out of the light path prior to the measurement so as to avoid incorrectness of the measurement due to absorption or scattering of light by the stirrer. However, such procedure is generally not favorable for assay of minute volume of samples.
Therefore, there is a continuing need for methods for assaying extremely small amounts of samples for their optical properties with high precision.